The present invention relates to a semiconductor device using a metal oxide thin film as a capacitor dielectric film and a method of manufacturing the same.
It is possible to allow an electronic device to perform a required function by improving the circuit construction. In addition, with recent progress in miniaturization of electronic devices, it is advantageous to use a functional thin film capable of utilizing the characteristics of the film material itself for achieving the required function.
Particularly, the cell area is now greatly diminished in integrated circuits including those using many transistors in combination for storing information such as SRAM (Static Random Access read write Memory) and EEPROM (Electrically Erasable and Programmable Read Only Memory), and those using transistors and capacitors in combination for storing information such as DRAM (Dynamic Random Access Memory) and FRAM (Ferroelectric Random Access read write Memory). It is now very difficult to form these integrated circuits by using the conventional MOS transistors and MOS capacitors.
Particularly, in order to prevent the S/N ratio from being lowered, a capacitor is required to maintain a predetermined capacitance even if the minimum processing dimension of the integrated circuit is diminished. As a material of a dielectric film of a capacitor meeting this requirement, attentions are paid to functional materials exhibiting a dielectric constant higher than that of a silicon oxide film or a silicon nitride/silicon oxide laminate film (NO film).
Metal oxides having a perovskite crystal structure such as Ba.sub.x Sr.sub.1-x TiO.sub.3 (BST) and PbZr.sub.x Ti.sub.1-x O.sub.3 (PZT) exhibit a dielectric constant of several hundred at room temperature and, thus, are useful as capacitor dielectric film. In using a thin film of the metal oxide as a capacitor dielectric film, it is impossible to use a conventional material of polycrystalline silicon (poly-Si) for forming an electrode, particularly, a lower electrode. If a metal oxide thin film is formed directly on the poly-Si electrode, the surface of the poly-Si electrode is oxidized to form a SiO.sub.2 layer having a small dielectric constant. It follows that the effective amount of charge accumulated in the capacitor is determined by the capacitance of the SiO.sub.2 layer.
To overcome the above-noted difficulty, a noble metal such as platinum (Pt), iridium (Ir), ruthenium (Ru), rhenium (Re), or osmium (Os) is generally used for forming the lower electrode of a capacitor using a dielectric thin film of BST or PZT. Platinum is scarcely oxidized. The other noble metals such as Ir, Ru, Re and Os may be oxidized to some extent. However, the oxides of these noble metals act as conductors. Because of the low reactivity with oxygen, these noble metals are used for forming the electrode of a capacitor including a metal oxide dielectric thin film.
As pointed out above, ruthenium, even if oxidized, forms a conductive oxide. In addition, ruthenium tetraoxide (RuO.sub.4) has a high vapor pressure, making it possible to subject a Ru electrode to dry etching widely employed nowadays in the manufacture of a semiconductor device. It is also possible to connect a Ru electrode directly to a contact plug generally made of poly-Si without interposing a barrier metal. Such being the situation, Ru is considered to be the most prospective electrode material.
However, because of the low reactivity, Ru fails to adhere strongly to the poly-Si contact plug or to a base layer such as an interlayer insulating film. As a result, the Ru film tends to peel in the subsequent heating step or sintering step. In addition, Ru has a high melting point unlike other noble metals such as Pt. It follows that a Ru film, if formed under low temperatures, has a low crystallinity. If a heat treatment is applied to a Ru film having a low crystallinity, a change in stress accompanying a change in the crystallinity of Ru causes the Ru film to peel more easily from the under layer or from the dielectric film formed on the Ru film.
As described above, a Ru film exhibits excellent characteristics if used as a lower electrode of a capacitor including a metal oxide dielectric thin film. However, since Ru is low in reactivity, the Ru film fails to adhere strongly to a base layer and tends to peel. It follows that reliability of the capacitor including a metal oxide dielectric thin film is lowered.